Chemical measurements made on-site can be very effective in underpinning environmental decisions, but they are often mistrusted in favour of measurements made on samples removed off-site to a remote, usually accredited,\ud laboratory. The uncertainty present in all measurements, wherever they are made, includes contributions from the sampling and sample preparation processes, and also from often unsuspected systematic errors. Once this total uncertainty has been estimated in a rigorous way under statistical control for each investigation, it is demonstrated using two case studies that on-site measurements can be fit for decision-making purposes. Uncertainty from sampling and sample preparation often dominates many measurement systems, whether they are based on-site or off-site, and makes the analytical contribution less critical for judging fitness. The value of this total uncertainty can be used to make a probabilistic, rather than deterministic, classification of the contamination.\ud Uncertainty values can also be used to calculate how the measurementmethod can be modified to achieve an optimal value that it fit-for-purpose (e.g. using composite samples ormeasurements). The rapid availability of on-site measurements, together with known uncertainty, is shown to be capable of enabling equal reliability of decisions to the off-site approach, whilst minimising the cost of the decision making process. Challenges in estimating the uncertainty of on-site measurements are identified as (i) potential ambiguity in the true value that is being estimated (i.e. the measurand), and (ii) the fact that off-site lab measurements can have values of analytical precision that are larger than those quoted by the lab (e.g. 54% rather than 30% for TPH, at 95% confidence)\ud and which make their use in the validation of on-site measurements problematic.